1. Field of the Invention:
This invention relates to a single crystal pulling system of the type including electromagnets which apply a magnetic field to a single crystal material melt.
2. Description of the Prior Art:
Presently, in the semiconductor device industry there are primarily two methods for producing a single crystal Si or Ga-As. One method is the Czochralski method, and the other method is Free-Zone method. Most of single crystal LSI wafer materials are produced by the Czochralski method.
A conventional single crystal pulling system for the Czochralski method will be described.
A crucible filled with a single crystal material melt (hereinafter simply referred to as melt) is heated by a heater such that the single crystal material is invariably maintained in a melt state. A seed crystal is inserted into the melt, and when the seed crystal is pulled from the melt at a specific constant speed by means of a pulling mechanism, a single crystal is created through growth of the seed crystal at a boundary layer of the crystal-melt interface. In this process, liquid movements, i.e., thermal convections induced by heat of the heater, are somewhat developed.
Why such thermal convections are developed may be explained as follows. Thermal convections generally occur when the balance between buoyancy caused by fluid thermal expansion and fluid viscosity is destroyed. The balance between the buoyancy and the viscosity can be expressed by a dimensionless quantity which is called the Grashof number NGr, as follows: EQU NGr=g.alpha..DELTA.T R.sup.3 /.nu..sup.3
where:
g: gravitational acceleration PA0 .alpha.: thermal expansion coefficient of the melt PA0 .DELTA.T: radial temperature difference in the crucible PA0 R: crucible radius PA0 .nu.: dynamic viscosity coefficient of the melt PA0 .mu.: magnetic permeability of the melt PA0 H: magnetic field strength PA0 D: crucible radius PA0 .sigma.: electrical conductivity of the melt PA0 .rho.: density of the melt.
In general, when the Grashof number exceeds a certain critical value determined by various factors, such as geometrical dimensions of the melt, thermal boundary conditions and so forth, thermal convections occur within the melt. Usually, the thermal convections of the melt attain a turbulent flow state when NGr&gt;10.sup.5, and a disturbance state when NGr&gt;10.sup.9. In the case of the present melt condition under which a single crystal with a diameter of 3 to 4 inches is pulled, the Grashof number becomes NGr&gt;10.sup.9 (according to the above-described equation for NGr). As a result, there is developed a disturbance state within the melt, and a ruffled state is developed at the surface of the melt, i.e., at the crystal-melt boundary interface layer.
In the presence of the thermal convections of such disturbance state, temperature fluctuations within the melt, particularly at the crystal-melt boundary interface layer, become drastric. In turn, there exist drastic fluctuations in position and time elapse at the crystal-melt boundary interface. Consequently, microscopic remelting of the crystal during growth conspicuously occurs, and within the grown single crystal, there are developed dislocation loops, lamination defects and so forth. Further, such defective portions are developed in a non-uniform fashion with respect to the pulling direction of the single crystal because of irregular fluctuations at the crystal-melt boundary interface layer. Moreover, impurities are resolved from the inner surface of the crucible into the melt due to the chemical change between the crucible and the melt (particularly a high temperature melt of approximately 1500.degree. C., for example) which are in contact with each other. Such impurities are carried by the thermal convections resulting in an entire dispersion throughout the melt.
The impurities become nuclei, and within the single crystal, there are developed dislocation loops, lamination defects, growth stripes and so forth, whereby the quality of the single crystal is deteriorated. Therefore, in the process of manufacturing LSI wafers from the single crystal, the wafers that include such defective portions exhibit deteriorated electrical characteristics, to the point where they become useless and the production yield is therefore lowered.
In the future, single crystals are increasingly required to be greater in diameter. However, as can be seen from the equation for the Grashof number, the greater the crucible radius, the greater the Grashof number, so that the thermal convections of the melt become more violent. Thus the quality of the single crystal further deteriorates.
In recent years, it has been proposed to apply a direct current magnetic field to the melt in order to suppress the above-described thermal convections, thereby allowing single crystals to be pulled under a growth condition that is thermally and chemically close to the equilibrium state, as described in "NIKKEI ELECTRONlCS" (1980.9.15, pp. 154-176).
FIG. 1 shows a schematic configuration of the conventional single crystal pulling system utilizing application of a magnetic field.
In FIG. 1, a crucible 2 filled with a single crystal material melt 1 (hereinafter simply referred to as a melt) is heated by a heater 3 such that the single crystal material is invariably maintained in a melt state. A seed crystal 4 is inserted into the melt 1, and when the seed crystal 4 is pulled from the melt 1 at a specific constant speed by means of a pulling mechanism 5, a single crystal 7 is created through growth of the seed crystal 4 at the boundary layer of the crystal-melt interface 6.
In the outer periphery of a crucible 2 there is installed an electromagnet 10 so as to apply a uniform magnetic field to the melt 1 in the direction 9. The melt 1 for a single crystal 7 is generally a fluid conductor having an electrical conductivity .sigma., so that when such fluid moves by the effect of thermal convections 8, the fluid moving in a direction which is not in parallel with the direction 9 undergoes magnetic resistive force according to Lenz's law, and this prevents the movement of the thermal convections 8.
In general, magnetic resistive force obtained by application of magnetic field, i.e., magnetic viscosity coefficient .nu..sub.eff, can be expressed as follows: EQU .nu..sub.eff =(.mu.HD).sup.2 .sigma./.rho.
where
As can be seen from this equation, the greater the magnetic field strength H, the greater the magnetic viscosity efficient .nu..sub.eff. This causes .nu. in the aforementioned equation for the Grashof number to be increased, thereby causing the Grashof number thereof to be drastically decreased. Thus a certain strength of magnetic field can reduce the Grashof number to a value below the specified critical value. Consequently, the thermal convections 8 of the melt 1 are completely suppressed. Suppression of the thermal convections 8 by the virtue of application of magnetic field minimizes impurities within the single crystal 7, development of dislocation loops, and development of defective growth stripes. This also serves to create the single crystal 7 of uniform quality with respect to the pulling direction, thereby enhancing the quality of the single crystal 7 and its production yield as well.
FIG. 2 shows the relationship between the magnetic field strength (abscissa) and the concentration of impurities within the single crystal 7 (ordinate). In FIG. 2, when the magnetic field strength becomes greater than H.sub.1, the impurity concentration commences to decrease and becomes minimum at a certain magnetic field strength such as H.sub.2. This is because at the magnetic field strength H.sub.2, the Grashof number of the melt 1 becomes below the critical value, whereby the thermal convections 8 of the melt 1 are completely suppressed. Thus, even when the magnetic field strength is increased greater than H.sub.2, the impurity concentration is not further lowered because the thermal convections 8 have already been suppressed. It is useless to increase the magnetic field strength greater than H.sub.2.
As described above, higher concentration of impurities causes dislocation loops and defective growth stripes. Therefore, in order to create a single crystal of high quality, the concentration of impurities should be maintained within a hatched portion between the lines B.sub.1 and B.sub.2 shown in FIG. 2.
On the other hand, when the single crystal 7 is being pulled, the melt 1 within the crucible 2 decreases by the quantity spent for growth of the single crystal 7, so that the crystal-melt interface 6 lowers. To invariably apply a magnetic field of strength above H.sub.2 to the crystal-melt interface 6 in the process of pulling the single crystal 7, there is installed an electromagnet 10 capable of applying a magnetic field of strength above H.sub.2 to the entire space of the crucible 2. Thus, at the center of the crucible 2 where magnetic field strength becomes maximum, there exists a magnetic field of H.sub.3 &gt;H.sub.2 and the margin of such magnetic field strength becomes excessive. The electromagnet 10 is therefore, required to be relatively greater in field strength, and also adds further expense to the manufacturing cost. Further, because of its relatively greater magnetic field strength, the affected region thereof becomes greater. As a result, the magnetic field leakage of the electromagnet 10 adversely affects the pulling mechanism 5, particularly on electric motors and the like mounted therein.
Furthermore, the single crystal pulling system necessitates cleaning of the crucible 2 at every termination of pulling the single crystal 7 before replenishment of the crucible 2 with a new single crystal material. However, the conventional single crystal pulling system has such a configuration that the electromagnet 10 is fixed in the outer periphery of the crucible 2, so that when cleaning the inside of the crucible 2, the electromagnet 10 needs to be removed from the system, and this necessitates cumbersome and complicated procedures.